Saturday 9 May 2009 07.01 EDT
First published on Saturday 9 May 2009 07.01 EDT

The first time I was in front of a camera I wasn't self-conscious because I didn't really know what I was doing. I played the Porsche-driving boyfriend in a series called After Henry with Prunella Scales, Joan Sanderson and Janine Wood. I got out of a car, walked up to a house and got chucked. I played the scene as if I was in the theatre, without artifice.

I trained as a theatre actor at the Bristol Old Vic and learned about stage craft. In front of the camera, however, you have to hide all that technique. It has to become second nature.

I love the precision that film offers. You can convey things with tiny movements; it's much more like real life.

I was watching a film recently and there was a scene with one character comforting another. But instead of concentrating on comforting them, they let their eyeline drift up towards the camera. So, in effect, you were watching them showing you how comforting they were being.

Stop observing yourself. Find the truth in the situation instead. If you're in the middle of a dialogue, listen to the other actor, don't be thinking: is the light catching me in a gorgeous way?

When I put a character together, I do a lot of background research. But most of the work is actually instinctive. Sometimes you might imagine a scene first, only to realise, once on set, that you've got it completely wrong. For example,in The Way Back [a film about escaping from the Siberian gulags, directed by Peter Weir] there's a scene I play with Jim Sturgess and we had both imagined playing it inside, looking out through a window. As it turned out, we were outside in a blizzard, in a phalanx of about 100 extras, marching into the gulag, being completely battered by snow and wind. I literally could hardly get my words out. I was just trying to get through it, let alone keep my character intact.

What makes someone a great actor? Different people love different people. A great actor has to be intelligent and adaptable; they have to have humility and determination.